In recent years, the trend toward micro-scale pattern rule has been increasing with the trend toward large-scale integration and high-speed of LSI. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it has become possible to mass-produce DRAM (dynamic random-access memory) of 64M-bit (processing dimension is 0.25 μm or less) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Furthermore, in order to realize the production of DRAM's having integration degrees of 256M and 1G or greater, a lithography using ArF excimer laser (193 nm) has been used.
As a resist suitable for such exposure wavelength, “chemically amplified resist material” attracts much attention. This contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”) which generates an acid by radiation irradiation (hereinafter, referred to as “exposure”), and is a pattern-forming material that forms a pattern by making a difference in solubility in developing agent between the exposed portion and the unexposed portion through a reaction using the acid generated by the exposure as a catalyst.
Also concerning the photoacid generator used for such a chemically amplified resist material, studies have been made variously. It has been known that, in a case where the photoacid generator as has been used for a conventional chemically amplified resist material whose light source is KrF excimer laser light to generate alkane or arenesulfonic acid is used as a component of the above-mentioned ArF-type chemically amplified resist material, the acid strength is not sufficient to cleave an acid-unstable group so as not to allow resolution entirely, or that the sensitivity is so low as to make it unsuitable for the device production.
Therefore, as the photoacid generator for the ArF-type chemically amplified resist material, those that generate perfluoroalkanesulfonic acid high in acid strength are commonly used; however, perfluorooctane sulfonic acid and derivatives thereof, which are known as PFOS abbreviated by their initials, causes problems of stability (non-degradability) stemmed from a C—F bond and of biological concentration and accumulation stemmed from hydrophobicity or lipophilicity. Additionally, perfluoroalkanesulfonic acid having 5 or more carbon atoms and derivatives thereof also begin to cause the above problems.
In order to address the above problems regarding PFOS, there has been developed, at all parts, partially fluorinated alkanesulfonic acids having a reduced degree of fluorine substitution. For instance, alkoxycarbonylfluoromethanesulfonic acid onium salts such as triphenylsulfonium methoxycarbonyldifluoromethane sulfonate (Patent Document 1), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethane sulfonate (Patent Document 2) and triphenylsulfonium (adamant-1-ylmethyl)oxycarbonyldifluoromethane sulfonate (Patent Document 3) have been developed as the acid generator.
On the other hand, triphenylsulfonium 1,1,3,3,3-pentafluoro-2-benzoyloxypropane-1-sulfonate, which is a kind of alkylcarbonyloxyalkanesulfonic acid onium salt and has an ester bond opposite to that of the above-mentioned alkoxycarbonyldifluoromethanesulfonic acid onium salt, and the like have been developed (Patent Document 4).
The present applicant has found a 2-alkylcarbonyloxy-1,1-difluoroethanesulfonic acid onium salt represented by the formula [5] or [10] and having three less fluorine atoms than the acid generators presented by the Patent Documents so as to be considered to less affect the environment even though identical with the alkylcarbonyloxyalkanesulfonic acid onium salt. Additionally, the present applicant found this substance to function as an acid generator having a high acid strength with the minimum possible number of fluorine atom, and to be excellent in compatibility with solvents or resins so as to be useful as the acid generator for the resist material, and therefore already filed patent applications (Japanese Patent Application No. 2007-143879 and Japanese Patent Application No. 2007-143880).
By the way, as a method for synthesizing the above-mentioned alkoxycarbonyldifluoromethanesulfonic acid onium salt, a reaction path as represented by the following conventional equation [1]
has been known. More specifically, the path starts from synthesizing 3,3,4,4-tetrafluoro-[1,2]oxathietane 2,2-dioxide [iii] in the first place from tetrafluoroethylene [i] and sulfur trioxide [ii], and includes: synthesizing [v] by a ring-opening reaction of [iii] with the use of alcohol (ROH) or passing an acid fluoride [iv] through ring-opening isomerization of [iii] and then passing esterification of [iv] with the use of alcohol (ROH); subsequently converting [v] into a sulfonic acid (a sulfonic acid sodium salt) [vi] with the use of a basic metal salt (mainly sodium hydroxide); and thereafter conducting an onium-salt exchange with the use of an onium salt (Q+X−: Q is a monovalent onium cation and X is mainly halogen) such as a sulfonium salt thereby obtaining the target acid generator alkoxycarbonyldifluoroalkanesulfonic acid onium salt [vii] (Patent Document 1 and Patent Document 5).
On the other hand, as a method for synthesizing a 1,1,3,3,3-pentafluoro-2-benzoyloxypropane-1-sulfonic acid onium salt discussed in Patent Document 4, a reaction path as represented by the following equation [2] is disclosed.

However, hitherto known methods for producing a 2-alkylcarbonyloxy-1,1-difluoroethanesulfonic acid salt have been very few, which means that hitherto known methods for producing 2-(alkylcarbonyloxy)-1,1-difluoroethanesulfonic acid onium salt have been very few.    Patent Document 1: Japanese Patent Application Publication No. 2004-117959    Patent Document 2: Japanese Patent Application Publication No. 2002-214774    Patent Document 3: Japanese Patent Application Publication No. 2004-4561    Patent Document 4: Japanese Patent Application Publication No. 2007-145797    Patent Document 5: U.S. Pat. No. 2,852,554    Non-Patent Document 1: Solid State Ionics, 1999, Volume 123, pages 233-242